Polymeric foams are made using a blowing agent that generates a cellular structure as polymerization of reactants takes place. Trichloromonofluoromethane (CFC-11) has been the most often used physical blowing agent for various isocyanate-based polymeric foams, such as polyurethane foams, polyisocyanurate foams, polyurea foams, and polyamide foams, the hybrid foams, as well as for thermoplastic foams, e.g., polystyrene foams, polyethylene foams, polypropylene foams and phenolic foams.
However, due to the well-recognized ozone depletion problem in the stratosphere, all kinds of chlorofluorocarbons (CFCs) must be phased out by the end of 1995 according to a worldwide regulation. To achieve that end, some CFC-free, alternative physical blowing agents have been developed in recent years, albeit with limited success.
Chemical (or reactive) blowing agents such as water, which generates carbon dioxide gas by the reaction with isocyanate groups, constitute one class of such CFC-free blowing agents. However, water has the following disadvantages as the blowing agent for rigid foams: a relatively high exotherm which may cause scorching, high system viscosity, higher K-factor, poor adhesion to metal substrates due to thinner skin formation, lower dimensional stability (shrinkage), etc. In the case of flexible foams, disadvantages of water as a blowing agent include: relatively high exotherm which may cause discoloration and possible fire risk, and higher system viscosity. Furthermore, integral skin foam preparation is very difficult with such blowing agents, and therefore specific, costly polyols are required.
Other chemical blowing agents include enolizable compounds (K. Ashida, International Progress in Urethanes, Eds., Ashida, K. Frisch, K.C., Technomic Publishing Co., Inc. [1980], p. 153), polycarboxylic acids (U.S. Pat. No. 5,057,547 to Doerge), specific aldehydes or ketones (U.S. Pat. No. 5,079,271 to Gillis).
Physical (or non-reactive) blowing agents constitute another class of alternative blowing agents. Such physical blowing agents exhibit ozone depletion potential (ODP) and include the HCFCs (hydrochlorofluorocarbons, e.g., HCFC 141b, CCl.sub.2 FCH.sub.3) , the HFCs (hydrofluorocarbons, e.g., HFC-35b, C.sub.4 H.sub.4 F.sub.6) , the PFCs (pefluorohydrocarbons, e.g., PF-5050, C.sub.5 F.sub.12) and the HFCEs (hydrofluorocarbon ethers, e.g., E-245, CF.sub.3 CH.sub.2 OCH.sub.3). A physical blowing agent performs a blowing action by the exothermic reaction of polymer formation. Therefore, the lower the boiling point of the blowing agent, the lower the resulting foam density for the same mole amount of blowing agent. In order to obtain low density foams, a low boiling point liquid is desired. In addition, their raw materials costs are higher than those of other alternative blowing agents, and therefore, their potential for commercial applications is questionable.
Recently, n-pentane and cyclopentane have been used in European countries as physical blowing agents for rigid urethane foams (see, for example, E.E. Ball, Proceedings of the Polyurethanes World Congress 1993, Vancouver, Canada, Oct. 10-13, 1993, p. 10). However, due to their non-polarity or relatively low compatibility with other ingredients, special foaming formulations are required for this application.
Methylene chloride is another widely used blowing agent for flexible urethane foams, usually as a co-blowing agent with water. However, it is not suitable for use with rigid urethane foams due to a variety of processing problems, e.g., foam collapse or coarse cell foams. Therefore, no commercial applications have been reported to date.
A blend of methylene chloride and pentane also has been used for polyisocyanurate foam preparation (U.S. Pat. No. 4,898,893 to Ashida). However, this blowing agent encounters problems in preparation of rigid polyurethane foams, such as foam collapse, coarse cells, or open cell structure. Another type of physical blowing agent that has been used is liquid carbon dioxide (U.S. Pat. No. 5,120,770 to Doyle et al.). This blowing agent has a disadvantage in that a high pressure foaming equipment is required.
The use of acetone/cyclopentane combinations having a cyclopentane:acetone volume ratios between 85:15 and 60:40 as a blowing agent for rigid urethane foams is shown in U.S. Pat. No. 3,558,531 to Solyer. However, the obtained foam densities are undesirably higher than those of CFC-11 blown foams. Also known as blowing agents are halogen-containing azeotropes such as 2-methylbutane and HCFC141b (1,1-dichloro-1-fluoroethane), mentioned in U.S. Pat. No. 5,057,547 to Doerge, and such as trichlorofluoromethane and methyl formate disclosed in Proceedings of SPI 33rd annual Polyurethane Technical/Marketing Conference, Sept. 30-Oct. 3, 1990, p. 82. However, these latter two kinds of azeotrope contain halogens and thus have the drawback of presenting an ODP problem as well.